1. Field of the Invention
The present invention relates to an optical pickup that records and/or reproduces information to and/or from an information recording medium such as an optical disk, and an optical disk using the optical pickup.
2. Description of the Related Art
An optical pickup is well known in which liquid-crystal optical elements are used to correct spherical aberration, astigmatism and coma, respectively. For example, there is known a liquid-crystal optical element having a spherical aberration-correction electrode pattern 201 as shown in FIG. 1, a liquid-crystal optical element having an astigmatism-correcting electrode pattern 211 as shown in FIG. 2, a liquid-crystal optical element having a coma-correcting electrode pattern 221 as shown in FIG. 3, and an optical pickup including those elements.
The electrode pattern 201 has a plurality of concentric boundaries 202a and 202b that divide an electrode having the electrode pattern 201 provided thereon into a plurality of zones as shown in FIG. 1. The electrode pattern 211 has a circular boundary 212a and a plurality of radial boundaries 212b to 212i provided outside the boundary 212a, these boundaries dividing an electrode having the electrode pattern 211 provided thereon into a plurality of zones as shown in FIG. 2. Also, the electrode pattern 221 has a plurality of boundaries 222a to 222d that divide an electrode having the electrode pattern 221 provided thereon into a plurality of zones as shown in FIG. 3.
The liquid-crystal optical element includes two electrodes and a layer of liquid crystal molecules aligned between the two electrodes. Each of the two electrodes has formed thereon any of the above-mentioned electrode patterns. By controlling a voltage applied to the electrode pattern, the alignment of the liquid crystal molecules can be biased correspondingly to an electric field arisen by the applied voltage to change the refractive power, thereby correcting a desired one of the aberrations.
In such an optical pickup, since the liquid-crystal optical element includes only two electrodes, the electrode pattern can correct only two of the spherical aberration, astigmatism and coma as selected. Namely, since one liquid-crystal optical element cannot cover all such aberrations, another liquid-crystal optical element has to be additionally provided to correct all the aberrations.
However, addition of the liquid-crystal optical element to the optical pickup adds to the number of parts and makes it necessary to provide a circuit that drives the additional liquid-crystal optical element and the like. Therefore, the optical pickup including the increased number of parts will be complicated in structure and expensive.
Also, there is an optical pickup including a liquid-crystal optical element having an electrode pattern that corrects two aberrations, for example, spherical aberration and astigmatism.
In the optical pickup, however, since a pattern to correct the spherical aberration and astigmatism is formed on one of the electrodes of the liquid-crystal optical element, radial and circumferential parting lines have to be provided on the outer surface of the electrode.
Thus, in such an optical pickup, the number of zones resulted from division by the electrode pattern is increased and each of the zones has to be driven by controlling both the amounts of correction of spherical aberration and astigmatism. Namely, the number of zones to be driven independently and thus a complicated drive circuit is required to drive the liquid-crystal optical element.
Also, an optical pickup is proposed in which a transparent liquid-crystal electrode is zoned vertically and horizontally to correct aberrations (as in the Japanese Patent Application JP 1997-128785).
In such an optical pickup, however, the liquid crystal drive circuit is complicated and hence cannot be controlled in any simple manner. Also, since the transparent electrode is divided in many zones, the number of boundaries between the zones is increased, resulting in a lower efficiency of aberration correction.